The present invention relates to a flat panel image sensor being capable of detecting an image of X-rays and other kind of radiation and visible, infrared, and other kind of light.
Conventionally known flat panel image sensors are arranged so that while a semiconductor film, detecting X-rays and generating electric charges imparted by electron-hole pairs, is provided on an active matrix substrate, electric switches are provided in pixels arranged on the active matrix substrate in a matrix manner, and the electric switches are sequentially turned on in pixels of each column to read electric charges of pixels in each row.
For instance, the structure and principles thereof are explained in publications including L. S. Jeromin, et al., xe2x80x9cApplication of a-Si Active-Matrix Technology in a X-Ray Detector Panelxe2x80x9d (SID 97 Digest, pp. 91-94, disclosed on May 13, 1997) and W. den Boer, et al., xe2x80x9cSimilarities between TFT Arrays for Direct-Conversion X-Ray Sensors and High-Aperture AMLCDSxe2x80x9d (SID 98 Digest, pp. 371-374, disclosed on May 17, 1998).
Now, an arrangement of the conventional flat panel image sensor will be discussed.
FIG. 12 shows that a conventional flat panel image sensor 100 includes:
an active matrix substrate 200 which is substantially square and on which pixels 210 are arranged in a matrix manner;
a substantially square semiconductor film 300, sharing the center with the active matrix substrate 200, formed on the active matrix substrate 200; and a bias electrode 400 formed on the substantially entire surface of the semiconductor film 300.
Each pixel 210 on the active matrix substrate 200 includes, as an enlarged view of FIG. 13 shows, electrode wires arranged in an XY matrix including a scanning wire 211 and a signal wire 212, a thin film transistor (hereinafter, will be simply referred to as TFT) 213, and a charge storage (hereinafter, will be simply referred to as CS) 214 formed with respect to the active matrix substrate 200 (see FIG. 12).
The semiconductor film 300 (see FIG. 12) is made of a photoconductive substance that generates electric charges when X-rays and other kind of radiation are projected thereon. Photoconductivity means a characteristic that electric charges are generated when X-rays etc. are projected.
For instance, in the documents cited above, what is employed for use as the semiconductor film is amorphous selenium (hereinafter, will be simply referred to as a-Se) that has high dark resistance, exhibits satisfactory photoconductivity when exposed to X-rays, and is easy to form large films by vapor deposition.
Meanwhile, an active matrix substrate formed in a process to manufacture a liquid crystal display unit can be appropriated for use as the active matrix substrate 200. For instance, since each pixel of the active matrix substrate used for an active matrix liquid crystal display (hereinafter, will be simply referred to as AMLCD) includes a TFT made of amorphous silicon (hereinafter, will be simply referred to as a-Si) or polysilicon (hereinafter, will be simply referred to as p-Si), XY matrix electrodes, and a CS, the active matrix substrate is easily modified as the active matrix substrate for the flat panel image sensor.
Next, a function of a conventional flat panel image sensor will be described. When X-rays and other kind of radiation are projected onto the semiconductor film 300 such as an a-Se film, electric charges are generated in the semiconductor film 300. The charges generated in the semiconductor film 300 move toward the anodes and cathodes, if voltage is applied to the bias electrode 400. Consequently, the electric charges are accumulated in the CS 214 formed on the active matrix substrate 200.
The electric charges thus accumulated in the CS 214 can be taken out to the outside through the signal wire 212, by making the TFT 213 into an on-state by means of an input signal from the scanning wire 211.
As FIG. 13 indicates, the electrode wire including the scanning wire 211 and the signal wire 212, the TFT 213, the CS 214 etc. are all provided on the active matrix substrate 200 (see FIG. 12) in an XY matrix manner. Thus electric charges as information of images accumulated in each CS 214 are taken out to the outside through the associated signal wire 212 by making the associated TFT 213 into an on-state by sequentially scanning an input signal with respect to the associated scanning wire 211, so that information on a two-dimensional image of X-rays is obtainable.
Moreover, if the semiconductor film 300 of the conventional flat panel image sensor 100 exhibits photoconductivity to visible or infrared light, as well as radiation such as X-rays, the flat panel image sensor 100 can function as a two-dimensional visible or infrared image detector. For instance, since the a-Se film mentioned above exhibits a satisfactory level of photoconductivity to visible light, the film can be used as a high sensitive image sensor by utilizing avalanche effect on the application of a strong electric field.
However, the conventional flat panel image sensor 100 could discharge around the sensor due to a requirement of charging high voltage to the bias electrode 400. Moreover, the semiconductor film 300 is susceptible to pollution.
To resolve these problems, as FIG. 14 shows, a conventional flat panel image sensor 110 includes:
a spacer 500 which is shaped like a substantially square frame and provided on an edge of an active matrix substrate 200;
an insulating resin 600 formed to cover both areas on the active matrix substrate 200, one which is indicated as Y10 on which the semiconductor chip 300 is formed, and the other which is indicated as X10 on which the semiconductor chip 300 is not formed; and
a protective substrate 700 provided to be in parallel with the active matrix substrate 200 while keeping a certain distance from the substrate 200 by the spacer 500.
Incidentally, the insulating resin 600 is sealed by the spacer 500, and the protective substrate 700 and the active matrix substrate 200 are a substantially identical square.
The protective substrate 700 provided above the insulating resin 600 is situated for:
improving the strength of the flat panel image sensor 110;
mechanically protecting an exposed surface of the insulating resin 600; and
segregating the insulating resin 600 from outside moisture, etc.
However, in the conventional flat panel image sensor 110, the semiconductor film 300 is arranged to have the thickness around from several hundred microns to 1 mm, to improve absorption efficiency of X-rays. Therefore, in the part Y10 where the semiconductor film 300 is formed on the active matrix substrate 200, the thickness of the insulating resin 600 formed on the active matrix substrate 200 differs from the thickness in the part X10 where the semiconductor film 300 is not formed.
In other words, a thickness x10 of the insulating resin 600 in the area X10 is greater than a thickness y10 of the insulating resin 600 in the area Y10, because the thickness equivalent to that of the semiconductor film 300 is added to the former.
Furthermore, the volume of resin materials such as a photo-setting resin, a thermosetting resin, and a two-liquid-setting resin shrinks around 5 to 10% when hardened.
For instance, assuming that the thickness of the semiconductor film 300 is 1 mm, the difference between the thicknesses x10 and y10 of the insulating resin 600 is 1 mm, and hence the difference in the reduction in thickness of the insulating resin 600 due to the hardening differs by around 50 to 100 xcexcm.
In this manner, the area Y10 on the active matrix substrate 200 where the semiconductor film 300 is formed differs from the surrounding area X10 where the semiconductor film 300 is not formed, in the amount of the reduced thickness of the insulating resin 600 due to the hardening. Thus great internal stress is accumulated around the border of the areas Y10 and X10.
As a result, the conventional flat panel image sensor 110 is not reliable in terms of the strength for various reasons: namely the accumulated stress causes the edge of the active matrix substrate 200 being warped and impact strength of the flat panel image sensor 110 being degraded, etc.
The present invention has an object to provide a highly reliable flat panel image sensor in terms of the strength, to resolve the problems described above.
To achieve the object, a flat panel image sensor of the present invention includes:
an active matrix substrate;
a semiconductor film formed on the active matrix substrate;
a bias electrode formed on the semiconductor film;
a first spacer formed on the active matrix substrate so as to surround the semiconductor film;
an insulating resin formed on surfaces of the semiconductor film, the bias electrode, and the active matrix substrate, and sealed by the first spacer; and
a protective substrate facing the active matrix substrate via the first spacer and covering the insulating film,
wherein the insulating resin formed on the semiconductor film is as thick as the insulating resin formed on a peripheral part of the active matrix substrate, in which part neither the semiconductor film nor the first spacer is formed.
In this invention, the insulating resin formed on the semiconductor film is as thick as the insulating resin formed on a peripheral part of the active matrix substrate, in which part neither the semiconductor film nor the first spacer is formed. In other words, the thickness of the insulating resin in the both sides of the border, i.e. in the part on the active matrix substrate, in which part the semiconductor film is formed, and in the part of the active matrix substrate, in which part the semiconductor film is not formed, is uniform.
The inventor compared the flat panel image sensor of the present invention arranged as above with a conventionally structured flat panel image sensor, by examining an internal stress in the insulating resin around the border between a part of the active matrix substrate in which part the semiconductor film is formed and in a part of the active matrix substrate in which part the semiconductor film is not formed. Incidentally, the examined conventionally structured flat panel image sensor shares a common arrangement with the flat panel image sensor of the present invention, except that the thickness of an insulating resin formed on semiconductor film is different from the thickness of an insulating resin formed on a surrounding part of the active matrix substrate, in which part the semiconductor film and a first spacer are not formed. That is, the difference of the thickness of the insulating resin around the border of these two parts is observed in the latter.
Tremendous effort has proven that the internal stress generated around the aforementioned border in the flat panel image sensor of the present embodiment, in which no difference of the thickness of the insulating resin was observed around the border, is smaller than the internal stress around the border in the conventionally structured flat panel image sensor, in which the difference was observed.
Therefore the stress in the insulating resin can be reduced and hence the warpage of the active matrix substrate is prevented in advance, and the impact strength of the flat panel image sensor is improved.
As a result, it is possible to provide the flat panel image sensor having excellent strength.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.